This invention relates to the coating of articles and, more particularly, to an economical, effective approach for coating aircraft gas turbine parts to reject heat and thereby reduce thermal fatigue failures to help meet life objectives without using additional cooling air.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by an axial-flow compressor, and mixed with fuel. The mixture is combusted, and the resulting hot combustion gases are passed through an axial-flow turbine. The flow of gas turns the turbine by contacting an airfoil portion of the turbine blade, which in turn provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
A continuing trend is to increase the operating temperature of the gas turbine engine, as higher temperatures lead to greater thermodynamic efficiency. The ability of the engine to operate at ever-higher temperatures is limited by the materials used in the engine. A variety of techniques are used to achieve increased operating temperatures of the materials. Improved materials with inherently higher operating temperatures are developed. New processing techniques, such as directional solidification and improved heat treatments are utilized. Bleed-air cooling by air directed from the compressor to the hot sections of the engine is widely used.
Coatings are also important contributors to the increased temperature capability of modern gas turbine engines. Environmental coatings inhibit corrosive damage to the coated articles, allowing them to operate in environments, such as the high-temperature corrosive combustion gas, for which they would otherwise be unsuited. Ceramic thermal barrier coatings, usually overlying environmental coatings that serve as bond coats, serve as insulation layers.
Another type of coating is a layer of an optically reflective material that reflects a portion of the incident radiative heat loading away from the coated article. This type of heat-reflective and heat-rejection coating may be made of a metal or a ceramic adhered to the surface of the protected article. The drawbacks of these coatings are that they are relatively expensive to apply and may adversely affect the properties of the underlying substrate article upon which they are deposited. Additionally, it is difficult to apply the coatings to large articles.
There is a need for an approach to applying heat-reflective coatings that may be readily and inexpensively utilized both for newly made and repaired/refurbished articles, and which does not adversely affect the underlying substrate articles upon which the coatings are applied. The present invention fulfills this need, and further provides related advantages.
The present invention provides a heat-rejection coating that is readily and inexpensively applied to a metallic substrate article such as a superalloy article. The heat-rejection coating aids in preventing heating of the article by reflecting incident radiant energy. The present approach is readily utilized with large articles that do not fit into the deposition chambers required for prior approaches.
A method of applying a heat-rejection coating comprises the steps of supplying a metallic component of a gas turbine engine, air-assisted spraying a reflective-coating mixture onto the component, the reflective-coating mixture comprising a metallic pigment and a carrier, and firing the component having the reflective-coating mixture thereon to form a reflective coating on the component.
The component to which the reflective coating is applied is most preferably made of a nickel-base superalloy. Examples of components to which the coating may be applied include an exhaust nozzle convergent flap, an exhaust nozzle convergent seal, and parts that experience radiation in the combustion and exhaust systems.
The reflective-coating mixture desirably includes the metallic pigment in the form of finely divided particles and/or metal salts which precipitate atomic size metal particles of a metal such as platinum, gold, palladium, or alloys of these metals such as platinum-rhodium alloys. The carrier is a liquid that allows the metallic pigment to flow through the air-spray system and then aids in initially adhering the metallic pigment to the surface of the component prior to firing. Organic carriers are preferred.
The reflective coating is quite thin, both to conserve the expensive metal and to avoid a coating that adversely affects the properties of the underlying component. Because the reflective coating is thin, it is preferred to specify its quantity by a real weight rather than by thickness. Most preferably, the reflective coating is present in an amount of from about 0.00275 to about 0.00475 grams per square inch of the component surface being cooled.
Preferably, the component surface is pre-treated prior to the application of the reflective-coating mixture, so that the reflective-coating mixture is air sprayed onto the pre-treated surface. Pre-treatments include one or more of (a) polishing the component surface, (b) pre-oxidizing the component surface, (c) vapor depositing an oxide barrier coating, and (d) applying a ceramic barrier coating onto the component surface and thereafter drying the ceramic barrier coating. Most preferably, pre-treatments (a), (b), and (d) are used together, in the indicated order.
The ceramic barrier coating, where used, is preferably applied by air-assisted spraying a ceramic-barrier-coating mixture onto the surface of the component, and then drying the ceramic-barrier-coating mixture to form the ceramic barrier coating. The ceramic barrier coating is preferably supplied as the ceramic-barrier-coating mixture of particles of a ceramic material such as lanthanum and cerium, dispersed in a ceramic-barrier coating carrier such as an organic liquid. The ceramic barrier coating, where used, is desirably thin. The reflective coating and the ceramic barrier coating are preferably together present in an amount of from about 0.00325 to about 0.0625 grams per square inch of the component surface being cooled.
The reflective coating and, preferably, the ceramic barrier coating are both applied by air-assisted spraying. Air-assisted spraying is a technique comparable to the familiar spraying of ordinary paint, and is typically performed at room temperature using an air-spray-gun type of device. The material to be sprayed, here the reflective coating and possibly the ceramic barrier coating, are not significantly heated during the spray process (although they are heated subsequently in the firing step). Air-assisted spraying is to be contrasted with other spray techniques used to deposit other types of coatings in the gas turbine industry, such as vacuum plasma spraying and air plasma spraying, which are not within the scope of the invention. Plasma spray techniques are performed by heating the material to be sprayed to high temperatures and then forcing the heated material against the surface with a flow of the spray gas. Air-assisted spraying is also to be contrasted with other types of deposition techniques such as chemical vapor deposition, physical vapor deposition, and electrodeposition, all of which require complex deposition apparatus, and all of which are not within the scope of the invention. Most of these other application techniques are limited as to the size of the articles that may be readily coated, because they require special chambers or other types of application apparatus. Air-assisted spraying, on the other hand, is not limited by these considerations, and therefore may be readily used on a wide variety of sizes and shapes of components.
Other room-temperature application techniques such as airless spray, brushing, and application by a decal transfer method may also be used in the present approach.
The present approach may be used to deposit an alloyed metallic coating, as distinct from a pure metallic coating.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.